so it was working before and has now stopped sending the email? I would suspect an issue with the smtp server, check with your host to see if they can look into the smtp server error logs to see why the emails are not being relayed.

when testing the form, what are you entering for the emailAddress? make sure it is not the same email that is used for the To Address or the BCC address. the host will refuse to relay the message if the from and to address are the same.

when testing the form is it redirecting to the thankyou.php page?

the x-mailier is an optional setting, it is used to set what the mailing client is, Outlook, Thunderbird, etc.